1. Field of the Invention
This invention relates generally to medical devices, and, more particularly, to implantable medical devices for providing various types of therapies to patients.
2. Description of the Related Art
A cardiac pacemaker (i.e., pacemaker) is an implantable medical device that delivers electrical stimulation (i.e., xe2x80x9cpacingxe2x80x9d) pulses to cardiac tissue. Pacemakers are typically used to relieve symptoms associated with bradycardia, a condition in which patients cannot normally maintain physiologically acceptable heart rates. A wide variety of pacemakers are known and commercially available.
Early pacemakers delivered pacing pulses at regular intervals (i.e., constant rates) to maintain preselected heart rates. The preselected heart rate was typically deemed appropriate when the patient was at rest. Such pacemakers were known as xe2x80x9casynchronousxe2x80x9d pacemakers because they did not synchronize pacing pulses with natural cardiac activity.
In contrast, the heart rate of a typical healthy person with a properly functioning heart increases during periods of elevated physical activity, and decreases during periods of reduced physical activity, to meet changing metabolic and physiologic needs. Accordingly, the metabolic and physiologic requirements of a patient receiving therapy via a pacemaker producing pacing pulses at a constant rate are typically not met when the patient is engaged in physical activity. During periods of elevated physical activity, the patient may experience adverse physiological consequences, including lightheadedness and/or episodes of fainting.
To reduce the adverse effects of constant rate pacing, xe2x80x9crate responsivexe2x80x9d pacemakers have been developed that automatically adjust patients"" heart rates to meet changing metabolic and physiologic demands. In a typical rate responsive pacemaker, the rate at which pacing pulses are produced (i.e., the xe2x80x9cpacing ratexe2x80x9d) is variable between predetermined minimum and maximum rates. The minimum and maximum rates may be, for example, selected and programmed into the pacemaker by a physician. A xe2x80x9ctargetxe2x80x9d pacing rate of a rate responsive pacemaker may be expressed as:
Target Pacing Rate=Minimum Rate+ƒ(sensor output) 
where ƒ is a linear or monotonic function of an output of a single sensor, or the combined or xe2x80x9cblendedxe2x80x9d outputs of multiple sensors.
Some known rate responsive pacemakers include only a single xe2x80x9cactivityxe2x80x9d sensor (e.g., a piezoelectric crystal). In this situation, the rate response function ƒ is function of the activity sensor output. When the output of the activity sensor indicates that the patient""s activity level has increased, the pacing rate is increased from the minimum rate by an incremental amount, which is determined as a function of the output of the activity sensor. As long as the activity sensor output indicates patient activity, the target pacing rate is periodically increased by incremental amounts calculated according to the above formula, until the maximum rate is reached. When patient activity ceases, the target pacing rate is gradually reduced, until the minimum rate is reached.
For any rate responsive pacemaker, it is desirable that the activity sensor output correlate to as high a degree as possible with the metabolic and physiologic needs of the patient, such that the pacing rate determined by the activity sensor output meets the metabolic and physiologic needs of the patient. It is noted that activity sensor output only indirectly represents a level of metabolic need. In addition, physical activity sensed by an activity sensor can be influenced by upper body motion. For example, an exercise involving arm motion may result in an activity sensor output corresponding to a relatively high level of metabolic need, while the actual level of metabolic need is much lower. Conversely, an exercise that stimulates the lower body only, such as bicycle riding, may result in an activity sensor output corresponding to a relatively low level of metabolic need, while the actual level of metabolic need is much higher.
Other known types of rate responsive pacemakers include multiple sensors, and the rate response function ƒ may be a function of an output of one or more of the multiple sensors at any given time. For example, a rate responsive pacemaker may include an activity sensor and a xe2x80x9cminute ventilation sensor.xe2x80x9d Minute ventilation (Vc) is a parameter that has been demonstrated clinically to correlate directly to the actual metabolic and physiologic needs of a patient. Minute ventilation may be defined by the equation:
Vc=RRxc3x97VT 
where RR is a xe2x80x9crespiration ratexe2x80x9d in breaths per minute, and VT is a xe2x80x9ctidal volumexe2x80x9d of each breath in liters. Clinically, the measurement of Vc is performed by having the patient breathe directly into a device that measures the exchange of air and computes the total volume per minute.
While it is not possible for an implanted device, such as a pacemaker, to directly measure minute ventilation, it is possible for such an implanted device to measure impedance changes in the thoracic cavity. It is well known that a change in thoracic impedance corresponds to a change in tidal volume (VT), and a frequency of such changes over time corresponds to respiration rate (RR). (See, for example, U.S. Pat. No. 4,702,253 issued to Nappholz et al. on Oct. 27, 1987.) In a rate responsive pacemaker, circuitry configured to measure thoracic impedance, to extract respiratory rate (RR) and tidal volume (VT) values from thoracic impedance measurements, and to produce an output that represents a product of the respiratory rate (RR) and tidal volume (VT) values may be considered a xe2x80x9cminute ventilation sensor.xe2x80x9d
Both respiration rate (RR) and tidal volume (VT) have inherent physiologic time delays due to the response of CO2 receptors and the autonomic nervous system. As a result, an increase in minute ventilation (Vc) occurs after the onset of exercise and lags behind a need for increased cardiac output.
In rate responsive pacemakers having multiple sensors, rate response function ƒ may be selected such that the pacing rate is based on the combined or xe2x80x9cblendedxe2x80x9d outputs of the multiple sensors. For example, known rate responsive pacemakers include an activity sensor and a xe2x80x9cminute ventilation sensorxe2x80x9d as described above. In such rate responsive pacemakers, the rate response function ƒ may be selected such that the pacing rate is based substantially (or even solely) on the activity sensor output when the patient is relatively inactive, and based substantially on the output of the xe2x80x9cminute ventilation sensorxe2x80x9d when the patient is relatively active.
Human sleep-wake cycles are examples of biological rhythms called xe2x80x9ccircadian rhythmsxe2x80x9dxe2x80x94internally originating cycles of behavior or biological activity with a period of about 24 hours. It is believed that human sleep-wake cycles are generated by an internal clock that is synchronized to light-dark cycles in the environment and other daily cues.
While the typical healthy person with a properly functioning heart is awake but relatively inactive, the person""s heart rate is usually at a xe2x80x9cresting rate.xe2x80x9d When the person is sleeping, the person""s heart rate typically drops to a xe2x80x9csleeping ratexe2x80x9d that is less than the resting rate. On the other hand, the heart rate of a patient receiving therapy via a typical rate responsive pacemaker is maintained at the above described minimum rate when the patient is both awake but relatively inactive and sleeping. While the difference between the xe2x80x9cresting ratexe2x80x9d and the xe2x80x9csleeping ratexe2x80x9d may be relatively small (e.g., about 5 beats per minute), the inability of the typical pacemaker to reduce the patient""s heart rate when the patient is sleeping may cause the patient to have difficulty falling asleep and/or sleeping well. In addition, since it is likely that the patient could tolerate, and even benefit from, a lower heart rate while sleeping, the pacemaker may be viewed as wasting limited energy reserves by maintaining the unnecessarily high minimum rate while the patient is sleeping.
Pacemakers are known that include an internal clock for keeping track of time and having a xe2x80x9csleep timexe2x80x9d function, wherein when the xe2x80x9csleep timexe2x80x9d function is enabled, the above described xe2x80x9ctargetxe2x80x9d heart rate for a patient receiving therapy via the pacemaker is reduced to a xe2x80x9csleep rate,xe2x80x9d which is typically lower than the programmed xe2x80x9cminimum rate,xe2x80x9d during a xe2x80x9csleep periodxe2x80x9d between a programmable xe2x80x9cbed timexe2x80x9d and a programmable xe2x80x9cwake time.xe2x80x9d A problem arises, however, in that the above timekeeping method is not optimal when the patient changes his/her bed time and/or wake time, travels to a different time zone, etc.
The present invention is directed to a method that may solve, or at least reduce, some or all of the aforementioned problems, and systems incorporating the method.
An implantable medical device system is described including an implantable medical device for implantation in a patient. One embodiment of the implantable medical device includes a therapy component, a minute ventilation sensing circuit, and computational circuitry coupled to the therapy component and the minute ventilation sensing circuit. The therapy component provides a therapy to the patient. The minute ventilation sensing circuit produces minute ventilation values indicative of a minute ventilation of the patient at time intervals. The computational circuitry receives a number of the minute ventilation values over a period of time, calculates a central tendency (e.g., a mean) of the minute ventilation values, and calculates a deviation of the minute ventilation values from the central tendency (e.g., a standard deviation of the minute ventilation values). The computational circuitry detects an onset of sleep in the patient when the deviation of the minute ventilation values from the central tendency is less than a predetermined minute ventilation threshold value, and signals the therapy component to modify the therapy when the onset of sleep is detected in the patient. For example, where the computational circuitry calculates a standard deviation of the minute ventilation values, the computational circuitry may detect the onset of sleep in the patient when the standard deviation of the minute ventilation values is less than the minute ventilation threshold value.
The implantable medical device may also include an activity sensing circuit producing activity values indicative of an activity level of the patient at time intervals, and the computational circuitry may be coupled to receive the activity values. The computational circuitry may detect the onset of sleep in the patient when: (i) the deviation of the minute ventilation values from the central tendency is less than the predetermined minute ventilation threshold value, and (ii) an activity value indicative of a current level of activity of the patient is less than an activity threshold value.
Further, the computational circuitry may be configured to keep track of a time of day. The computational circuitry may detect the onset of sleep in the patient when: (i) the deviation of the minute ventilation values from the central tendency is less than the predetermined minute ventilation threshold value, and (ii) an activity value indicative of a current level of activity of the patient is less than an activity threshold value, and (iii) a current time of day is greater than or equal to an expected sleep time value, wherein the expected sleep time value is indicative of a time of day the patient is expected to go to sleep.
The implantable medical device may be, for example, an implantable pacemaker, and the therapy component may be a pacing output unit of the pacemaker. The pacing output unit may be configurable to provide electrical stimulation to a portion of a heart of the patient dependent upon a low rate limit value, wherein the low rate limit value specifies a minimum rate of sensed contractions of the portion of the heart. The computational circuitry may detect an onset of sleep in the patient as described above, and may reduce the low rate limit value when the onset of sleep is detected in the patient.
A method is disclosed for providing therapy to a patient, including detecting an onset of sleep in the patient, and modifying the therapy following the detecting the onset of sleep in the patient. In one embodiment, detection of the onset of sleep includes: (i) receiving multiple minute ventilation values over a period of time, wherein the minute ventilation values are indicative of a minute ventilation of the patient, (ii) calculating a central tendency of the minute ventilation values, (iii) calculating a deviation of the minute ventilation values from the central tendency, and (iv) detecting the onset of sleep in the patient if the deviation of the minute ventilation values from the central tendency is less than a predetermined minute ventilation threshold value.
As described above, the calculation of the central tendency may include calculating a mean of the minute ventilation values, and the calculating the deviation of the minute ventilation values from the central tendency may include calculating a standard deviation of the minute ventilation values. The onset of sleep may be detected in the patient if the standard deviation is less than the minute ventilation threshold value.
The method may also include receiving an activity value indicative of a current level of activity of the patient. In this situation, the onset of sleep may be detected in the patient if: (i) the deviation of the minute ventilation values from the central tendency is less than the predetermined minute ventilation threshold value, and (ii) the activity value is less than an activity threshold value. Alternately, onset of sleep may be detected in the patient if: (i) the deviation of the minute ventilation values from the central tendency is less than the predetermined minute ventilation threshold value, and (ii) the activity value is less than the activity threshold value, and (iii) a current time of day is greater than or equal to an expected sleep time, wherein the expected sleep time is a time of day the patient is expected to go to sleep.
In one embodiment of the method, the detecting the onset of sleep in the patient involves receiving a first number of the minute ventilation values over a first period of time. The first period of time may be, for example, greater than or equal to 24 hours. The first number of the minute ventilation values is used to determine a minute ventilation threshold value. A second number of the multiple minute ventilation values are received over a second period of time following the first period of time. A central tendency of the second number of minute ventilation values is calculated, as is a deviation of the second number of minute ventilation values from the central tendency. The onset of sleep is detected in the patient if the deviation of the second number of minute ventilation values from the central tendency is less than the minute ventilation threshold value.
The using the first number of minute ventilation values to determine the minute ventilation threshold value may include, for example, receiving a portion of the first number of minute ventilation values during each of multiple time intervals of the first period of time. At the end of each time interval, the following may be calculated: (i) a central tendency of the minute ventilation values received during the time interval, and (ii) a deviation of the minute ventilation values received during the time interval from the central tendency. A histogram may be formed reflecting the deviations of the minute ventilation values received during the time intervals from the central tendencies. A pair of peaks may be located in the histogram. A minute ventilation value residing between the peaks in the histogram may be selected as the minute ventilation threshold value.
For example, at the end of each time interval, the following may be calculated: (i) a mean of the minute ventilation values received during the time interval, and (iii) a standard deviation of the minute ventilation values received during the time interval. In this situation, the histogram reflects the standard deviations of the minute ventilation values received during the time intervals.